Rapidly growing solid tumors are known to shed circulating tumor cells (CTCs) that, in turn, then enter the bloodstream3-4. CTCs can produce metastatic tumors at sites remote from the primary tumor, and these secondary tumors may be the source of deadly metastatic disease. Early, sensitive detection of CTCs may provide an avenue to early cancer diagnosis5. Furthermore, a better understanding of CTCs' genetic makeup may inform the development of advanced treatments against metastatic cancer.
The very low abundance of CTCs in whole blood—where they are outnumbered by blood cells by about a billion to one—makes the isolation and analysis of CTCs challenging. Achieving more efficient, specific capture of CTCs may therefore be desirable in the field of nanobiotechnology.
There has been interest in development of fluidic devices for CTC capture6-14. Approaches based on affinity capture6-8,15, magnetic sorting9, and size-based separation16-17 have been reported, often used in conjunction with imaging or off-chip conventional gene expression profiling methods that are used for characterization. However, the very low levels of CTCs present in patient blood samples typically necessitate that several milliliters of whole blood be processed. Thus, it may be desirable to achieve sufficient throughput, as well as sufficient capture yield.
Magnetic nanoparticles (also referred to as nanobeads) have been investigated for targeting CTCs. Magnetic nanobeads can be made specific to CTCs through the attachment of an antibody against a cell-surface marker, for example. Because thousands of nanobeads can attach to a cell18, this approach may allow specific targeting of CTCs within the large blood samples that may need to be processed for highly sensitive analysis. Magnetic nanobeads, unlike magnetic microbeads19-21, may offer stability in solution over the time intervals typically needed to process a typical whole blood sample.
Combining magnetic nanobeads with sufficiently efficient and practical fluidic separation has, to date, been unsuccessful, possibly because magnetic nanobeads typically possess a low inherent magnetic susceptibility18,21-22. Practical magnetic fields that can be applied in a fluidic device are, when combined with the typical low magnetic susceptibilities of the nanobeads, likely incapable of overcoming the drag forces produced by even slowly flowing liquids. A common method for trapping magnetic microparticles involves placing a permanent magnet or an electromagnet close to the microfluidic channel13. However, trapping of sub-100 nm nanobeads typically requires high-gradient magnetic fields that can be difficult to achieve in compact devices.
It has been shown that patient CTCs are relatively heterogeneous, and that it is specific subpopulations with different gene expression profiles that tend to give rise to metastases. Therefore, it would be useful to have more straightforward methods for gene expression-based CTC sorting.23-24 